Amur tiger (Panthera tigris altaica) and Amur leopard (P. pardus orientalis), the flagship species of the ecosystems, mainly prey on red deer (Cervus elaphus), sika deer (C. nippon), roe deer (Capreolus pygargus), and wild boar (Sus scrofa) in the national park region, while the ungulates rely on forest understory plants. All form the major parts of the grazing food chains in the temperate forest ecosystem of Northeast Asia. The main objective of this study is to comprehend the understory plant resource of the area, and establish baseline data through a vast investigation of the forest vegetation in the eastern part of the national park, in order to facilitate the estimation of the carrying capacity of the system in supporting the prey populations. In the growing seasons of 2015 and 2016, 141 sample locations associated with the established camera-trap platforms.were randomly selected The 496 transects were nested with the sampling locations, and 1948 sampling plots further nested with the transects. In this nested sampling system, understory biomass, plant group (tender twigs/leaves, grasses/sedges, forbs, and ferns) compositions were investigated, and nitrogen and carbon contents of plant groups were tested. We found that, (1) Forest canopy closure influences the light availability of the understory vegetation, and its productivity and plant group composition. The understory biomass at the open sites was 3 times of that under the closed canopies (94.91 g/m² vs. 30.15 g/m²，P<0.01). (2) At the open sites, understory biomass was the highest at riparian areas, At the gaps, tender twigs/leaves mainly dominated, while at forest edges, forbs did so, and along the riparian area, mainly sedges. (3) The understory tender twigs/leaves and forbs had the highest N contents, forbs had the lowest C contents and therefore the lowest C/N ratios. The forbs and the tender twigs/leaves should be the better forages for the ungulates. (4) The calorific values of all four understory plant groups were above 17kJ/g with the forbs the lowest. (5) Based on the data, high understory biomass (>40 g/m²) plots dominated by the tender twigs/leaves and forbs were about 16 % of the close canopy plots and 50 % of the open site plots, which could be the hot forage sites for browsers. With the study results, we suggest to maintain some open habitats, as the supplement highly quantity and quality forage patch for wild ungulate i.e. sika deer, to boost their populations for facilitating restoration of the tiger and leopard.